Jack lies. Frequently. Routinely, even. He does it so much, he doesn't even realize he's doing it anymore. Even when he meets someone new and has to run through all his lies for the first time and swear up and down that he's telling them the truth. Of course, after someone's seen you survive being shot, point-blank, between the eyes and stand up without so much as a blemish, they tend to believe whatever whoppers you've got in store.
The thing about lies, though. They're tricky. Sometimes a lie works very easily… with most people. Sometimes, you get into a situation where you've set up a running lie that nobody even questions anymore, including yourself. Then you find yourself slipping suddenly into a situation in which that lie will be easily disproven… but you are too… well, caught up, to consider the implications.
That was how Ianto Jones learned that he wasn't the only one who wasn't entirely on the up-and-up. The first few nights, Jack always had some excuse. He'd invite Ianto to stay as long as he liked, but Jack always had a clandestine rendezvous to be off to. But eventually, Jack got comfortable and admitted he disliked rushing off, preferring a nice cuddle afterward.
Ianto wasn't sure how he'd felt about that at the time. When Jack was hurrying away, it felt less intimate and allowed him time to 'process' what he was doing while showering quickly before running down to the basement to check on Lisa. He tried not to show how tense it made him, Jack staying and snuggling him. He tried pretending to fall asleep, hoping Jack wouldn't want to do pillow talk.
Then came the time Ianto realized that Jack was even stiller than he was, but not tensely still. Jack was sound asleep. Jack, who always contended that he, in fact, did not sleep. That was also when Ianto realized how beautiful Jack really was. Not that flash-Jack - all teeth and charm and personality and billowing coat and good hair - that he flung exuberantly at the world every day. This sleeping Jack had an entirely different sort of beauty. Quiet and vulnerable and somehow, it seemed to Ianto, relieved to let the performance drop for a while.
But the main thing was Jack was asleep, and that meant 'I don't sleep' was a lie. Which made Ianto wonder what else might be. What else did Jack make up, insulate himself in to the point that he believed it himself?
The longer Ianto got to know Jack, the more he came to see that understanding Jack was a lot like watching him sleep. You had to learn to see what was secreted behind the shield of blinding brightness. You had to hear what went unsaid. You had to read between the lies. The trick was all in telling the difference between a lie that Jack had been telling so long he believed it and ones he thought he was telling to protect others.